A non-linear junction detector (hereinafter “NLJD”) is a device which can be used to detect junctions between two materials exhibiting dissimilar electronic or electrical properties. Examples of non-linear junctions are semiconductor junctions or junctions between dissimilar metals. Such detectors are widely used in counter surveillance operations and in electronic ordnance detection.
An exemplary operation of a prior art NLJD is illustrated in FIGS. 1 and 2.
The NLJD 101 operates by illuminating a target junction 102 with energy 103 at a fundamental RF frequency. Reflections 104, 105 from the non-linear junction 102 can then be analysed to determine the type of junction detected. The reflections from the non-linear junction usually have principal frequency components at twice and three times the illuminating signal frequency (2nd and 3rd harmonics).
In general, a semiconductor junction 102 will return predominantly second harmonics 104. Junctions between dissimilar metals 202 will return higher levels of third harmonic 205 or very similar levels of second and third harmonics 204, 205, but predominantly third harmonics.
In prior art systems, the power level of the transmitted illuminating signal has been manually determined by the operator. In general (maintaining all other NLJD system parameters constant) the detection range of the NLJD system 101, 201 can be altered by varying the level of the illuminating signal 103, 203.
In order to achieve maximum detection range the illuminating signal level 103, 203 must also be set to a maximum.
Consider a target junction 102, 202 with a fundamental to 2nd harmonic conversion loss of LR;LR=Pi−Po  (1)Where Pi is the illumination level at the target junction and Po is the reflected level at the 2nd harmonic.    ∴LR=PTJR−(2PTJR−k)  (2)    ∴LR=k−PTJR  (3)where k is a conversion constant; PTJR is the fundamental illuminating power level at the target junction and the factor of 2 assumes that the target junction conversion loss is a 2nd order function.
With reference to FIG. 3, assume that the target junction 302 is illuminated by a NLJD 301, also assume that the NLJD has a 2nd harmonic receiver 306 with detection sensitivity, S, and that the range between the target junction and the NLJD is R (km), see FIG. 3.
In order for the target junction 302 to be detected in free space the NLJD 301 must illuminate the junction with a power of PT,
 PT=S+B+LR+A  (4)
where A is the free space path loss between the NLJD transmitter and the target junction at the fundamental frequency, FREF(MHz),A=32.4+20log10(FREF)+20log10(R)  (5)and B is the free space path loss between the target junction and the NLJD 2nd harmonic receiver at the 2nd harmonic frequency,B=32.4+20log10(2×FREF)+20log10(R)=A+6  (6)
Using equations (3) and (6), the expression for PT (4) can be re-written as:PT=S+(A+6)+(k−PTJR)+A  (7)
The illuminating power level at the target junction can be expressed as:PTJR=PT−A  (8)
Substituting (8) into (7) gives the illumination level at the NLJD as:PT=S+(A+6)+(k−[PT−A])+A  (9)                              ∴                                           ⁢                                          ⁢                      P            T                          =                              S            +                          3              ⁢              A                        +            k            +            6                    2                                              (          10          )                ⁢                                       
Substituting (10) back into (8) gives the illumination level at the target as:                               P          TJR                =                              S            +            A            +            k            +            6                    2                                    (        11        )            
As a target junction 302 is approached by the detector 301, the level of the illuminating signal at the junction will also increase.
Consider now that the range between the target junction and the NLJD is reduced by a factor of 2 from R to R/2.
The level of the illuminating signal at the target junction, PTJR/2 will now increase to:
PTJR/2=PT−(A−6)  (12)                               ∴                                           ⁢                                          ⁢                      P                          TJR              /              2                                      =                              S            +            A            +            k            +            18                    2                                    (        13        )            
Therefore decreasing the range by a factor of 2 increases the illumination level at the target by:ΔPTJ=PTJR/2−PTJR=6 dB  (14)
In general, the increase in illumination level at the target junction results in an increase in the received harmonic reflection level.
Increasing the level at the target junction by ΔPTJ will decrease the junction conversion loss by a factor ΔPTJ (assuming that the target junction conversion loss is a second order function).
Therefore the target junction conversion loss will reduce to:
 LR/2=LR−(ΔPTJ)  (15)
Therefore the reflected 2nd harmonic level at the NLJD receiver, PR, will increase to:PR=PTJR/2−LR/2−A  (16)∴PR=PTJR/2−[k−PTJR/2]−A  (17)∴PR=S+18  (18)i.e. by reducing the range, R, by a factor of 2 the level at the NLJD 2nd harmonic receiver increases by 18 dB.
The increase in harmonic level can subsequently be used to indicate to the operator that the target junction is being approached.
Whilst the operating mechanism described previously is perfectly adequate for detecting the vast majority of non-linear junctions, increasing the illumination level at the target can have a number of undesirable effects.                a) The illumination level can increase to such an extent that the target junction becomes saturated and false indications can occur (e.g. semiconductor junctions can be indicated as metal to dissimilar metal junctions).                    Note: The 3rd harmonic response will increase at a rate proportional to 3 times the increase in illuminating signal level, whereas the 2nd harmonic response will increase at a rate proportional to 2 times the increase in illuminating signal level. This is illustrated in FIG. 4.                        b) System power (e.g. battery life) is wasted by allowing the level of the illuminating signal to increase beyond the point at which detection occurs.        c) In certain applications e.g. improvised explosive device detection (IED) or explosive ordnance disposal (EOD), an increase in illuminating signal at the target junction can lead to unwanted device triggering.        d) Interference to office equipment may occur which can alert others to the fact that an NLJD is in use.        e) Higher illumination levels increase the risk of interference to a casual observer.        